Chocolate fridge tart

Grease a 30 x 20 cm glass dish with butter or non-stick spray.
Heat the boiling water, sugar and butter together until the sugar has dissolved.
Make a smooth paste with the cornflour, cocoa, salt and water. Stir the cocoa paste into the sugar mixture and bring to the boil, stirring continuously.
Simmer over low heat until thick and cooked. Remove the mixture from the heat and stir in the vanilla essence.
Arrange a layer of biscuits in the bottom of the glass dish and spread with a layer of the chocolate mixture.
Repeat the layers until everything has been used, ending with a layer of the chocolate mixture. Leave until cool and set before sprinkling with the peppermint chocolate (if using).
Chill until needed.